(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of successfully modeling different end-point mode functions in order to maintain a constant and stable end-point detection curve for Chemical Mechanical Polishing (CMP).
(2) Description of the Prior Art
Inter-linear device distances have, over the years, become increasingly short due to the continued emphasis on semiconductor device performance improvements and device miniaturization. Semiconductor technological has evolved from Large Scale Integrated (LSI) devices to Very Large Scale Integration (VLSI) devices and ultimately to Ultra Large Scale Integration (ULSI) devices.
Photolithography is one of the predominant technologies that is used for the creation of semiconductor devices, photolithography advances are aimed at the creation of increasingly shallower focal depths that are needed for the creation of images in target surfaces. Due to the increasingly shallower focal depth, increased planarity of the target surfaces must be achieved. The reason for this is that a large step in a semiconductor surface, that is an abrupt change in the planarity of the surface, can have a severely negative impact on step coverage for the surface. If the gradation of the surface is too severe, poor deposition of for instance an overlying layer of metal can result, making it difficult to achieve a reliable semiconductor device. Further increases in semiconductor device density have frequently been achieved by implementing multi-layered configurations. This places further demands on the planarity of the surface over which overlying semiconductor device features are created. Optimum surface planarity has in recent years been obtained by applying methods of Chemical Mechanical Polishing (CMP) whereby semiconductor surfaces such as semiconductor substrates are uniformly polished to a high degree of planarity. The process is used to not only planarize semiconductor slices prior to the fabrication of semiconductor circuitry thereon, but is also used for the removal of high elevation features, which are created during the fabrication of the microelectronic circuitry on the substrate.
A brief overview will be given at this time of the CMP process. A CMP apparatus typically consists of a rotating polishing platen on the surface of which an abrasive polishing pad is mounted. A wafer is mounted on the surface of a second rotating part, the wafer carrier. The surface of the rotating wafer is the surface that is to be polished. The mounting of the wafer to the wafer carrier is frequently achieved by means of a clamp ring. The surface of the rotating wafer is brought in contact with the rotating polishing pad, a slurry is distributed over the surface of the rotating wafer. The chemical slurry, which frequently includes abrasive materials, is maintained on the polishing pad with the objective of modifying the polishing characteristics of the polishing pad, enhancing the polishing of the substrate.
The process of chemical mechanical polishing to planarize semiconductor substrates is not without problems. These problems are typically more pronounced where the process of CMP is used to remove high elevation features, which have been created during the fabrication of microelectronic circuitry on the surface of the substrate. One of the most serious problems that is experienced in using chemical mechanical polishing is the limited ability to predict or control the rate and uniformity at which material is removed from the surface that is being polished. CMP is therefore as yet a labor-intensive process, since surface thickness and uniformity must be frequently monitored during the process of CMP in order to prevent over-polishing or inconsistent polishing of the substrate surface.
Of special concern in this respect is the non-homogeneous replenishment of slurry on the surface of the substrate and the polishing pad. The slurry is primarily used to enhance the rate at which selected materials are removed from the substrate surface. Non-homogeneous replenishment of slurry on the surface that is being polished contributes to unpredictability and non-uniformity of the polishing rate of the CMP process. In view of the fact that a fixed volume of slurry, which is in contact with the surface that is being polished, reacts with materials on the surface that is being polished, this fixed volume of slurry becomes less reactive as the process of polishing proceeds. As a consequence, the polishing enhancing characteristics of the fixed volume of slurry are significantly reduced during polishing. To counter this problem, fresh slurry is continuously provided to the surface of the polishing pad.
This latter approach presents at least two problems. Because of the physical configuration of the polishing apparatus, introducing fresh slurry into the area of contact between the substrate and the polishing pad is difficult. Providing a fresh supply of slurry to all locations on the surface of the substrate is even more difficult. As a result, the uniformity and the overall rate of polishing are significantly affected as the slurry reacts with the surface of the substrate that is being polished.
FIG. 1 shows a Prior Art CMP apparatus. A polishing pad 12 is attached to a polishing table 16, rotating in a direction indicated by arrow 20 at a rate in the order of 1 to 200 RPM. A wafer carrier 14 holds wafer 10 (face down) against a polishing pad 12. Wafer 10 is held in place by applying a vacuum (not shown) to the backside of the wafer 10. Wafer carrier 14 also rotates, indicated by arrow 19, typically in the same direction as polishing table 16, rotating at a rate of about 1 to 200 RPM. The wafer traverses a circular polishing path over the polishing pad 12, due to the rotation of polishing table 16. Force 18 can also be applied against wafer 10 in the downward vertical direction and press the wafer 10 against the polishing pad 12 during the process of polishing. Force 18 is typically between about 0 and 15 pounds per square inch, force 18 is applied by shaft 17 that is attached to the back of wafer carrier 14. Slurry 13 is provided to the top of the polishing pad 12, this further enhances the polishing action of polishing pad 12.
The Prior Art method that is highlighted in FIG. 1 encounters a number of problems and concerns. For instance, abrasive polishing particles are lodged in and held by the polishing pad. By using the polishing pad, the fibers of the polishing pad deteriorate, causing abrasive particle retention within the polishing pad to diminish, reducing the polishing characteristics of the polishing pad. Also, due to the pressure that is applied to the wafer, the contact between the polishing pad and the wafer is intense and does not allow for an even distribution of the polishing slurry across the surface that is being polished. In addition, the abrasive particles that essentially affect the polishing action are, during the polishing process, reduced in size, further affecting the polishing characteristics of the process.
FIGS. 2a and 2b give an example of the process of Chemical Mechanical Polishing (CMP). The example that is shown in FIGS. 2a and 2b is purposely kept simple and can readily be expanded to more complex examples of CMP. For all of these examples, the polishing process is essentially as the example that is shown in FIGS. 2a and 2b. Over a semiconductor surface 10, for instance the surface of a silicon substrate, a layer 22 of for instance silicon nitride is deposited, patterned and etched, creating openings or trenches 21 through the layer 22 and into the underlying semiconductor surface. These trenches have been filled with an overlying layer 24, for instance comprising silicon oxide. It is the objective to fill the trenches 21 that have been created in layer 22 and the underlying semiconductor surface with a material. To achieve this objective, it is clear from FIG. 2a that the layer 24 must be removed from above the plane that contains the surface of layer 22. CMP is therefore applied to the surface of layer 24, removing this layer starting from the surface of the layer 24 and proceeding until the surface of layer 22 is reached. The results of the CMP process are shown in FIG. 2b, indicating that the objective of filling the trenches that have been created through layer 22 and into the surface 10 has been achieved. Numerous variations of the simplified example, shown in FIGS. 2a and 2b, of a processing environment where CMP is applied need not be further detailed at this time since these variations may change in particulars but not in the essential process as shown in FIGS. 2a and 2b. 
Where FIGS. 1, 2a and 2b have shown the principle of the methods that are used to implement surface polishing, an equally important aspect of surface polishing is to observe the status during and the results after the polishing. The invention provides a method that addresses semiconductor surface polishing.
U.S. Pat. No. 6,071,177 (Lin et al.) shows an endpoint method for a CMP process by utilizing a dual wavelength interference technique.
U.S. Pat. No. 5,949,927 (Tang), U.S. Pat. No. 5,948,203 (Wang) and U.S. Pat. No. 5,910,846 (Sandu) show other CMP endpoint methods.
It is an objective of the invention to successfully model different end-point mode functions in order to keep a constant and stable endpoint detection curve.
It is another objective of the invention to establish an optimum CMP pre-thickness of the layer that is being polished such that the end point failure rate of the CMP process for a layer of boro-phosphate-silicate-glass (BPSG) approaches zero.
It is a further objective of the invention to significantly improve the statistical measure of the process deviation from a process mean for the process of polishing a layer of BPSG using methods of CMP.
The polishing of a layer of boro-phosphate-silicate-glass (BPSG) is not easy to control as a result of the CMP slurry chemistry effects, the doping concentration of the layer of BPSG and the heat treatment to which the layer of BPSG has been submitted prior to the process of CMP. The invention has developed a CMP endpoint detection mode that minimizes variations on the process of CMP of a layer of BPSG by these factors. An endpoint detection algorithm has been developed, which has been applied and has proven to significantly improve a statistical measure, which indicates the process deviation from a process mean for the process of CMP of a layer of BPSG.